1. Field of the Invention
The present invention relates to a heat processing apparatus of the vertical type.
2. Description of the Related Art
The heat processing apparatus of the vertical type is usually used to carry out heat diffusing and film forming processes in the course of manufacturing semiconductor devices. This is because the heat processing apparatus of this type can keep the process tube free from dusts and also save the space which the apparatus occupies.
In the case of this heat processing apparatus, the heat processing furnace comprises a process tube shaped like a cylinder and made of quartz, and heaters and heat insulating material enclosing the process tube. The heat processing furnace thus arranged is erected and located in the upper half of a rectangular box. Formed in the lower half of the box is a space in which a wafer boat having on it a plurality of objects such as semiconductor wafers to be processed is held. The wafer boat thus held in the space is loaded and unloaded in and out of the heat processing furnace by a lifter means such as the boat elevator. The apparatus further includes a system for exhausting process gas and hot gas discharged from the heat processing furnace.
According to the heat processing apparatus of the vertical type arranged as described above, the space it occupies can be more saved and the total space for the semiconductor devices manufacturing system can be thus reduced to a greater extent, as compared with the heat processing apparatus of the horizontal type in which the heat processing furnace is set horizontal. In addition, the semiconductor wafers housed in the wafer boat can be made free from dusts because the wafer boat can be loaded and unloaded into and out of the heat processing furnace while being kept not contacted with the process tube.
In order to further keep the wafers not contaminated by dusts, there is also used another heat processing apparatus of the vertical type wherein a clean gas or air is allowed to flow through the space after passing through a dust removing filter, so as to prevent dusts from adhering to the wafers held in the space. The air which has passed through the space is discharged in this case into a maintenance room prepared, different from the clean room, to repair the heat processing apparatus.
In any case of these conventional heat processing apparatuses of the vertical type wherein the air once taken into the heat processing apparatus is discharged into the maintenance room, however, there cannot be avoided the possibility that particles discharged from the heat processing furnace may adhere to the semiconductor wafers. The semiconductor devices have been made smaller and smaller in size. In addition, the need of making them more highly integrated and producing them in a larger scale of mass production has become stronger and stronger these days. The adhering of dusts to the wafers, therefore, can add great influence to these tendencies because it can reduce the productivity of semiconductor devices. When attention is paid to the fact that the minimum width dimension of the semiconductor pattern is changing from 1.0 .mu.m to 0.5 .mu.m, it is strongly desired that the above-mentioned drawbacks will be eliminated.
Further, various harmful gases such as phosphine (PH.sub.3) and arsenic (As) are used as process gas at the time of heat process. Conventionally, these gases could be sufficiently exhausted by the exhaust system located at the open bottom of the heat processing furnace, because the amount of these gases used was relatively small and thus the amount of harmful gases remaining in the apparatus was extremely small. Therefore, no problem was caused even if the clean air which had passed through the space was discharged into the maintenance room.
As the semiconductor wafers become larger and larger in size, changing their size from 5 and 6 inches to 8 inches, however, the amount of gases used has become quite larger. This makes it impossible that sufficient exhaust is achieved only by the above-mentioned exhaust system. The influence of the harmful gases which will be added to the operator cannot be neglected, accordingly.
Further, the introducing of impurities into the semiconductor wafers or the doping of impurities such as P and As into them was also carried out by ion implantation systems. When the total cost and the manufacturing efficiency are taken into consideration, and when the difficulty of minimizing the semiconductor pattern in the order of microns because of heat diffusion caused by the annealing of the wafers after ion implantation is also taken into consideration, however, it has become these days that the impurity doping is carried out while conducting the film forming process. This is because the heat processing apparatus of the vertical type has been quite remarkably improved.
In the recent phosphor doping, quite high corrosive phosphorous oxychloride (POCl.sub.3) is used as process gas and it is introduced into the furnace by carrier gas such as oxygen or nitrogen gas. The heat process is carried out at a temperature of about 900.degree. C. in this case. When the heat process is finished, inactive gas is introduced into the furnace and gases remaining in the furnace are thus purged from the furnace. This purging of remaining gases is repeated several times. The wafer boat is then unloaded out of the furnace and held in the apparatus until the processed semiconductor wafers in it are cooled to about 200.degree. C.
Although POCl.sub.3 in the atmosphere in the furnace has been almost purged from the furnace in this case, POCl.sub.3 and others not reacted are still released from the surface of each of the processed semiconductor wafers. This POCl.sub.3 reacts with water or oxygen in the atmosphere to generate a reacted product in which phosphorous pentoxide is mainly contained. This reacted product containing mainly phosphorous pentoxide therein is liquid or solid depending upon its state generated. Particularly when it is liquid, its corrosiveness is quite so high as to easily corrode stainless steel. Various metal parts in the apparatus and means related to the apparatus can be thus eroded. When the reacted product containing mainly phosphorous pentoxide is left in the apparatus, therefore, micro-particles of corrosion float in the apparatus to adhere to the semiconductor devices, thereby lowering the productivity of the devices.